We have mapped numerous mouse and human genes to their specific chromosomes or chromosomal regions. We have also shown that the species-specific suppression of the production of rRNA in mouse-human hybrids is due to a block of the transcription of the mouse or the human genes for rRNA. We have also produced hybrids between totipotent mouse teratocarcinoma cells and either human or rat cells and shown that chimeric mice derived at least in part from the interspecies hybrids can be obtained.